The Problem
Tomasz Nowak is a freelance motion designer. He works on a desktop at home and on a laptop at client sites, and his project folder, ~/Projects, holds about 85 GB of After Effects files, fonts and exports. For two years he has carried it around on a USB drive and copied it by hand; twice he copied in the wrong direction and overwrote a day of work. Cloud sync would cost $20 a month for the storage he needs, and his clients' NDAs make him uneasy about putting footage on a third-party service. He also has an old mini-PC in a cupboard that could hold a safety copy.
He needs the two machines to stay identical without thinking about it, plus a copy on the mini-PC that nothing he deletes by accident can reach back and damage.
The Solution
Run Syncthing on all three machines. Desktop and laptop share the folder two ways. The mini-PC receives a copy of the same folder as receiveonly, so it never pushes changes back. Everything goes over encrypted device-to-device connections with no account. The agent drives Syncthing through its command line and REST API. The systemd skill keeps it running on the mini-PC, and ssh provides a tunnel to its GUI so the interface is never exposed to the network.
Step-by-Step Walkthrough
Step 1: Install and start Syncthing on each machine
Prompt: "Install Syncthing 2.x on this desktop, generate its keys, and tell me its device ID. Do the same on the mini-PC over SSH."
# distribution packages can be older; install the 2.x release as the syncthing skill describes
syncthing --version # must print v2.x, the cli subcommands below need it
syncthing generate
syncthing device-id
ssh minipc 'syncthing generate && syncthing device-id'
On the mini-PC the agent enables the packaged user service so it survives reboots:
ssh minipc 'sudo systemctl enable --now syncthing@tomasz.service && systemctl status syncthing@tomasz.service --no-pager | head -5'
The agent notes the three device IDs (desktop, laptop, minipc) and keeps them in shell variables DESKTOP_ID, LAPTOP_ID, MINIPC_ID.
Step 2: Pair the devices
Prompt: "On the desktop, add the laptop and the mini-PC as devices. Use a fixed address for the mini-PC, 192.168.1.50."
syncthing cli config devices add --device-id "$LAPTOP_ID" --name laptop --addresses dynamic
syncthing cli config devices add --device-id "$MINIPC_ID" --name minipc --addresses tcp://192.168.1.50:22000
The same two devices add calls, with the other IDs, are run on the laptop and on the mini-PC (which only needs the desktop and the laptop). After a few seconds syncthing cli show connections lists the paired devices with "connected": true.
Step 3: Share the folder, two ways between the computers and one way to the mini-PC
Prompt: "Share ~/Projects between desktop and laptop in both directions, and make the mini-PC receive-only."
On the desktop:
syncthing cli config folders add --id projects --label Projects --path "$HOME/Projects" --type sendreceive
syncthing cli config folders projects devices add --device-id "$LAPTOP_ID"
syncthing cli config folders projects devices add --device-id "$MINIPC_ID"
On the laptop, the same folders add with --type sendreceive and the desktop as device. On the mini-PC:
syncthing cli config folders add --id projects --label Projects --path /srv/backup/projects --type receiveonly
syncthing cli config folders projects devices add --device-id "$DESKTOP_ID"
The folder --id must match everywhere. The mini-PC never sends changes upstream, so a mistake there cannot damage the working copies.
Step 4: Keep deleted and replaced files on the mini-PC
Prompt: "If I delete something by mistake, the mini-PC should still have the old version for 30 days."
ssh minipc 'syncthing cli config folders projects versioning type set staggered'
ssh minipc 'syncthing cli config folders projects versioning params set maxAge 2592000'
ssh minipc 'syncthing cli config folders projects versioning dump-json'
The last command shows "type": "staggered" and "maxAge": "2592000" (seconds, 30 days). Files that sync would delete or overwrite on the mini-PC are moved into the folder's .stversions directory instead.
Step 5: Keep render caches out
Prompt: "Do not sync After Effects caches, render previews or OS junk."
export ST_URL=http://127.0.0.1:8384
curl -s -X POST -H "X-API-Key: $ST_KEY" -H "Content-Type: application/json" \
-d '{"ignore": ["(?d).DS_Store", "(?d)Thumbs.db", "Adobe After Effects Auto-Save", "*.aep.tmp", "/_cache"]}' \
"$ST_URL/rest/db/ignores?folder=projects"
ST_KEY comes from the Syncthing configuration of that machine. The same call is made on the laptop, because ignore patterns are per device.
Step 6: Confirm the first sync is complete
Prompt: "Tell me when the mini-PC has everything."
curl -s -H "X-API-Key: $ST_KEY" "$ST_URL/rest/db/completion?folder=projects&device=$MINIPC_ID"
The agent repeats the call until the answer contains "completion": 100 and "needItems": 0. With 85 GB on a gigabit LAN the first pass takes about 25 minutes.
Step 7: Look at the GUI safely when needed
Prompt: "I want to see the web interface of the mini-PC from my laptop."
ssh -N -L 8385:127.0.0.1:8384 minipc
The mini-PC's interface stays on its own loopback address; the tunnel makes it available at http://127.0.0.1:8385 on the laptop only for as long as the command runs. The agent also confirms with syncthing cli config gui dump-json that address is 127.0.0.1:8384.
Real-World Example
Tomasz finishes a title sequence on the desktop on Monday night and closes the lid. On Tuesday at a client's office he opens the laptop; the project is already there because the two devices synced as soon as the laptop joined the network. He edits the project, and the changes flow back to the desktop and to the mini-PC. On Thursday he deletes a folder of old exports by mistake on the laptop; the deletion reaches the desktop within seconds, but the mini-PC, which has file versioning switched on for the folder, still holds the old files in its .stversions directory and he restores them from there. He cancels the cloud storage subscription he never used up to the limit, and the USB drive goes into a drawer.